Food packaging films are composite laminate films, and are composed of various plastic films and/or metal foils having a thickness of approximately 5 to 100 μm. Examples of the plastic films include plastic films made of polyolefins such as polyethylenes and polypropylenes, copolymers of olefins, polyvinyl chlorides, polyvinylidene chlorides, polyesters, polyamides and the like. Examples of the metal foils include metal foils made of aluminum, stainless steel and the like.
These plastic films and metal foils are characterized by properties such as strength, water resistance, moisture permeability resistance, oxygen permeability resistance, and heat resistance. Therefore, high performance food packaging films, which cannot be obtained by using only a single film, can be provided by laminating two or more kinds of films, if necessary. Examples of adhesives used for producing the food packaging films include predominantly urethane adhesives. Patent Documents 1 to 3 disclose that a food packaging film is produced by laminating a plastic film using a urethane adhesive.
Patent Document 1 discloses an adhesive for food packaging films in which a carboxylic acid or an anhydride thereof, and an epoxy resin are mixed to form a urethane resin (see [claim 1] and [Examples] of Patent Document 1). Patent Document 2 discloses that a food packaging urethane adhesive is synthesized by mixing a polyisocyanate with a mixed polyol obtainable by mixing a polyester polyol having a glass transition temperature of 40° C. or higher with an organic polyol having a glass transition temperature of lower than 40° (see [claim 1] and respective Synthesis Examples of [Examples] of Patent Document 2).
Patent Document 3 discloses a urethane adhesive synthesized by mixing a polyol having a high number average molecular weight with a polyol having a low number average molecular weight, and mixing a polyisocyanate in the mixed polyol (see [claim 1] of Patent Document 3). In Examples, a polyol A is a polyester polyol, polyols B and C are polyetherpolyurethane polyols, and polyols D and E are polyesterpolyurethane polyols. A food packaging bag (or pouch) is produced by the urethane adhesive synthesized from these polyols and polyisocyanates, and appearance of the packaging bag after a boiling sterilization treatment is evaluated (see [0068], [Table 1] and [Table 2] of Patent Document 3).
Patent Document 4 discloses a urethane adhesive for laminates, comprising a silane coupling agent as an essential component (see [claim 1] of Patent Document 4). In Examples, a polyol A is a polyesterpolyurethane polyol, and polyols B and C are polyesterpolyurethane polyols. A composite film was produced by a urethane adhesive synthesized from these polyols and polyisocyanates, a bag was produced by using the composite film, and then the bag was subjected to steam sterilization and the peeling state (delamination) was evaluated (see [0090], [Table 1] and [Table 2] of Patent Document 4).
The object of Patent Documents 1 to 4 is to improve peel strength, heat resistance, and water resistance on boiling of the urethane adhesives, and each of these urethane adhesives of these documents is suitable for lamination of a food packaging film. However, in recent years, it is needed for the adhesive for food packaging films that appearance of the food packaging film is not adversely affected, even though a food packaging bag is produced, contents are put into the food packaging bag, the food packaging bag is subjected to sterilization and further stored over a certain period of time. In other words, the adhesive for food packaging films is required to be excellent in contents resistance. Considering workability in producing a packaging bag by using a film, it is important that the adhesive for food packaging films has appropriate viscosity to be easily applied, and is excellent in initial adhesion after lamination of a film.
The urethane adhesives of Patent Documents 1 to 4 exhibit peel strength improved by the addition of an epoxy resin or silane coupling agent and mixing of a plurality of polyols. However, the contents resistance and the initial adhesion are still insufficient, and it is necessary to further improve them.
Patent Document 1: JP 2683937 B
Patent Document 2: JP 3583629 B
Patent Document 3: JP 2003-129024 A
Patent Document 4: JP 4226852 B